Move to Dokku and webpack asset handling

[duffn]

Description

This is a gigantic PR to move to handling static assets as well as moving to Heroku. The static asset updates were based upon a fresh new 1.4 Phoenix project and its defaults

• Moves all static assets to /assets
• Combine a few CSS files into a single site.css
• Update all static assets to use Routes.static_path
• Update local development environment to handle webpack
• Add code reloading in dev.exs
• Update prod.exs to handle Heroku setup
• Add Dokku configuration files

TODO

• Write up checklist for Heroku switchover.
• Minify CSS files in vendor.
  Will be done by Cloudflare in Move DNS to Cloudflare #177.
• Update Docker setup to work with webpack.
• Update ansible-swsheets to work with this (if using DO).
  Will be using Dokku
• Update CORS settings in prod #150 needs to be resolved before this is deployed (if using DO).
  Will be using Dokku

Closes #127.

[codecov]

Codecov Report

Merging #137 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #137   +/-   ##
======================================
  Coverage    93.3%   93.3%           
======================================
  Files          47      47           
  Lines         558     558           
======================================
  Hits          521     521           
  Misses         37      37

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